Page 25 - Plastics News March 2021

FeAtures

Extrusion: Solving Surging in Two-Stage Screws

Jim Frankland

when the two stages are not matched in output against a particular head pressure, the second stage can con-
tinuously vary in fill length resulting in surging.

ver had a two-stage screw that is unstable all of the When the stages of a two-stage screw are not matched
Etime, regardless of temperature settings and screw in output against a particular head pressure, the second
speed? It happens when the two stages are not matched stage can vary continuously in fill length. This results
to the polymer output and die backpressure. in surging. One solution is to install a flow-control
valve at the discharge of the extruder and increase the
A two-stage screw is basically two separate screws that
must work in tandem. The first stage of the screw feeds backpressure until the cycling stops.
and melts polymer before dumping it into the vent area. In order for the second stage of the screw to be stable it
The second stage of the screw takes the melted polymer must have a minimum length of fill so that pressure flow
from the vent, pressurizes it, then pumps it through a die. is a small percentage of the drag flow. Otherwise, it will
The outputs of the two stages need to be relatively well continuously vary in fill length, which changes its output.
balanced or else constant instability will develop that As the fill length increases, the output of the second stage
can’t be solved by adjusting the operating conditions. A increases. As the fill length decreases, the output of the
vented or two-stage screw is essentially two screws on second stage decreases. So, when the two stages are not
the same shaft. The first works as a conventional screw matched in output against a particular head pressure,
by feeding, melting, and conveying the polymer forward. the second stage can continuously vary in fill length. This
The second screw does the same, except it is melt-fed results in surging. The surge will not be very noticeable in
by the first screw. The first screw operates with no head the power draw because most of the power is used in the
pressure, while the second has to overcome any head first stage for melting, with much less used in the second
pressure from the die and other downstream components. stage for melt conveying.
This situation has been known to happen when a two- For such situations, the best solution is to install a flow-
stage screw designed for a specific polymer is used to control valve at the discharge of the extruder and increase
run a different polymer. The result is a reduction in the the backpressure until the cycling stops, indicating enough
volumetric output of the first stage. This leaves the second fill to develop a stable condition. Alternatively, you might
stage too deep for the output of the first stage, and be able to solve this problem by adding filter screens
consequently the length of fill is altered. When designing or switching to a more restrictive breaker plate, but
a new two-stage screw with a good estimate for the head that’s not easily adjustable and can require continuous
pressure, the fill length can be calculated by manipulating retrofitting if the polymers or even bulk densities are
the depths to balance the drag flow and pressure flow to constantly changing. The accompanying illustration shows
obtain a reasonably stable length of fill. But when the the typical two-stage screw, compared with a two-stage
polymer being processed is changed, the optimum fill screw with too short a fill length and the screw with a
length may be irreversibly lost. flow-control valve to provide control over the fill length
regardless of the output from the first stage.

I’ve seen this problem crop up more frequently of late
because extruders are adding higher percentages of
recycled material to this process. The reduced bulk density
of the recycled material decreases the specific output
of extruders; a two-stage screw that worked well when
there was little or no regrind/recycle is now unstable. A
new screw design could correct this, but then you are still
not ready for the next polymer or a different bulk density.

25 March 2021 Plastics News